1. Field of the Invention
The present invention relates to a semiconductor laser excitation solid state laser apparatus capable of generating a laser beam stably and efficiently.
2. Description of the Related Art
FIGS. 17(a) and 17(b) are a side view and a cross sectional view, respectively, illustrating the construction of a conventional semiconductor laser excitation solid state laser apparatus disclosed for example in a document entitled xe2x80x9cAdvanced High-Power Lasersxe2x80x9d, SPIE Vol. 3889, pp. 182-189. FIG. 18 is a view illustrating the waveform of input current supplied to a semiconductor laser and the waveform of laser output power generated thereby.
In FIGS. 17(a) and 17(b), the conventional semiconductor laser excitation solid state laser apparatus includes an excitation light source in the form of a semiconductor laser 1 for irradiating semiconductor laser beam which is excitation light, a solid state laser element in the form of a slab-type Nd:YAG laser 2, and a flow tube 4.
The solid state laser element 2 is inserted into the flow tube 4. In order to cool the solid state laser element 2, a cooling medium 5 in the form of cooling water is caused to flow between the solid state laser element 2 and the flow tube 4. The semiconductor laser 1 is installed outside the flow tube 4, and irradiates a semiconductor laser beam 3 which is reflected and guided by a gold mirror 6 to pass through the flow tube 4 to excite the solid state laser element 2. In addition, the surroundings of the flow tube 4 are covered with the gold mirror 6 excepting an area thereof entered by the semiconductor laser beam 3. With this arrangement, there is obtained the effect of confining the semiconductor laser beam 3 inside the flow tube 4, thus improving the excitation efficiency. Also, provisions are made for a main reflection mirror and a partial reflection mirror which together constitute a resonator. A laser beam 11 is radiated from the resonator to the outside thereof.
Now, reference will be made to the operation of the solid state laser apparatus by means of the pulsed excitation of the semiconductor laser 1.
The semiconductor laser 1, when supplied with current, emits a semiconductor laser beam from its light emitting portion. In the past, when the semiconductor laser is caused to perform a pulsed operation, the current value within one pulse was maintained constant as shown by the input current waveform in FIG. 18. Inside the semiconductor laser, a difference between the output energy taken out therefrom as the semiconductor laser beam and the input energy due to the current supplied thereto results in the heat generated by the semiconductor laser. Though not shown herein, this heat is removed by means of an unillustrated cooling device.
The solid state laser element 2 is excited by the semiconductor laser beam irradiated from the semiconductor laser 1 to generate spontaneous emission light.
In this case, by the provision of the optical resonator composed of the mirrors 9 and 10, as illustrated in FIG. 17(a), the spontaneous emission light is amplified while repeatedly reciprocating between the mirrors 9, 10 of the resonator so that it is turned into a laser beam of a good directivity. When the laser beam is amplified to a certain magnitude or above, it is discharged as a laser beam 11 to the outside of the resonator.
Here, note that it takes a certain time from the start of current being supplied to the semiconductor laser 1 until the time a thermal equilibrium is established between the input energy and the output energy taken out as the semiconductor laser beam plus the discharged heat of the cooling device. Therefore, in cases where a constant current is always supplied to the semiconductor laser 1 as in the prior art, the internal temperature of the semiconductor laser 1 changes over time until the thermal equilibrium is reached inside the semiconductor laser 1.
The oscillation spectrum of the semiconductor laser 1 has temperature dependency and changes into a longer wavelength as the temperature rises. In the pulsed operation of the conventional semiconductor laser 1, the oscillation spectrum within one pulse also changes over time in accordance with a temperature change. As a result, the consistency between the oscillation spectrum of the semiconductor laser 1 and the absorption spectrum of the solid state laser element 2 decreases even in the entire duration of one pulse, thus resulting in a decrease in the absorption factor of the semiconductor laser beam, etc.
For example, in case where the oscillation spectrum of the semiconductor laser 1 is longer than the absorption spectrum of the solid state laser element 2 under thermal equilibrium, the absorption factor of the semiconductor laser beam in the solid state laser element 2 becomes high immediately after the start of excitation. However, the absorption factor of the semiconductor laser beam within one pulse decreases successively due to a wavelength change over time resulting from a temperature rise, so that the laser output power in a one pulse duration also decreases successively. Thus, when the semiconductor laser is applied to the usage of processing or the like, there arises a problem of deteriorated processing quality.
Moreover, since the absorption factor of the excitation light immediately after the start of excitation is high, a spike of the laser pulse head due to a relaxed oscillation (i.e., gradual oscillation) becomes remarkable as appearing in FIG. 18. This becomes not only a cause of deteriorating the processing quality but also one of factors damaging optical components such as resonator mirrors, etc.
The present invention is intended to obviate the problems as referred to above, and has its object to provide a semiconductor laser excitation solid state laser apparatus which is capable of suppressing, in a semiconductor laser under pulsed operation, a change in the oscillation spectrum of the semiconductor laser within one pulse, and exciting a solid state laser medium efficiently to generate a laser beam of high power in a stable manner.
Bearing the above object in mind, according to the present invention, there is provided a semiconductor laser excitation solid state apparatus comprising: a solid state laser element containing an active medium; a semiconductor laser for optically exciting the solid state laser element; a power supply for supplying electric power to the semiconductor laser; and an optical resonator for taking out a laser beam from the optically excited solid state laser element; wherein when the semiconductor laser is pulse-operated to pulse-excite the solid state laser element, current supplied to the solid state laser element is changed within one pulse.
In a preferred form of the present invention, when the semiconductor laser is pulse-operated to pulse-excite the solid state laser element, current supplied to the solid state laser element is decreased successively within one pulse.
In another preferred form of the present invention, when the semiconductor laser is pulse-operated to pulse-excite the solid state laser element, current supplied to the solid state laser element is decreased successively in an initial stage of a pulse within one pulse.
In a further preferred form of the present invention, when the semiconductor laser is pulse-operated to pulse-excite the solid state laser element, current supplied to the solid state laser element is increased successively within one pulse.
In a yet further preferred form of the present invention, when the semiconductor laser is pulse-operated to pulse-excite the solid state laser element, current supplied to the solid state laser element is increased successively in an initial stage of a pulse within one pulse.
In a still further preferred form of the present invention, when the semiconductor laser is pulse-operated to pulse-excite the solid state laser element, current supplied to the solid state laser element is changed stepwise within one pulse.
In a further preferred form of the present invention, the semiconductor laser excitation solid state laser apparatus further comprises: a diffusive reflector arranged to enclose the solid state laser element and having an inner surface constructed to diffuse and reflect laser beam; and an optical waveguide element for guiding the laser beam emitted from the semiconductor laser into the interior of the diffusive reflector while repeating total reflections of the laser beam.
In a further preferred form of the present invention, the solid state laser element has a rectangle cross section and is arranged on a cooling plate.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.